It is well known that runners are subjected to sever impacts upon foot strike. This can lead to trauma to the lower extremities, such as to the shin and foot, as well as to upper body parts, such as to the back.
In an attempt to lessen this trauma, many running shoes have been developed that have limited shock absorbency.
For example, it is known to use closed or open fluid-filled chambers, e.g., air or gel, in the sole of a running shoe. As a runner steps, the fluid compresses or is released from the chamber, which provides some cushioning to the runner. The amount of cushioning, however, is proportional to the amount of fluid contained in the chamber. For increased cushioning, the chamber must be made larger, and thus the clearance between the foot and sole must be increased. This decreases the stability of the shoe, particularly in the lateral direction, which can actually increase the likelihood of lower extremity injuries, such as shin splits. Thus, fluid-filled running shoes may not decrease trauma.
Moreover, running shoes with closed fluid-chambers do not significantly reduce the shock associated with foot strike. As a runner's foot strikes the ground, the fluid in these chambers compresses and firms up. This produces a shock to the runner as he or she steps. In addition, it is difficult to predict how much fluid pressure should be pre-filled in a chamber since one runner may be heavier or lighter than another runner. For example, for a lighter runner, the fluid will not sufficiently compress, and, therefore, will not provide much shock absorbency. On the other hand, for a heavy runner, the fluid may compress too much and the chamber may bottom out. Bottoming out may also occur because some runners are more aggressive or run faster that others, which can also lead to over-compression of the fluid in the chambers. Thus, it is difficult to design an inexpensive and reliable fluid chamber running shoe that reduces shocks for runners of different weights and running styles. Moreover, most manufacturers design and pre-fill such fluid chamber runner shoes for the average user, which means that the shoes do not provide sufficient cushioning for lightweight, heavyset or aggressive runners.
Other known designs use springs under the heel to provide cushioning as a runner steps. Springs, however, suffer from similar problems as fluid chambers in that it is difficult to install a spring that will provide adequate cushioning for runners of different weights and running styles. This is because springs under the heel, like fluid that compresses within a chamber, also compress and firm up, and thus may shock the runner. In addition, a spring, like fluid, needs clearance in which to compress, and, the space that the springs and chambers take up in the sole further reduces this clearance.
To increase cushioning, the clearance between the foot and sole must be increased, which decreases the stability of the shoe. Moreover, since different runners have different weights and different running styles, it is difficult to design a spring that is suitable for all runners.
Moreover, known devices fail to provide shock absorption throughout the full range of a runners step. More particularly, as a runners foot strikes the ground, the lower edge of the shoe strikes the ground first. As the shoe contacts the ground, the runners body weight, which has a forward and downward momentum, is forced against the lower heel of the shoe as the runner's foot suddenly decelerates. This force must be adequately cushioned to prevent a shock or jar to the runner.
After the initial impact, the lower leg then rotates over the shoe relative to the ground, and the runner rotates his body weight over the shoe. It is also important to provide cushioning in this range because, as the runner moves over the shoe, body weight is being transferred to the shoe. As the stride continues, the leg continues to rotate, and the forefoot strikes the ground. By the time the forefoot strikes the ground, most of the impact associated with the stride has already been absorbed by the runner.
Known devices, however, fail to provide adequate cushioning during the entire range of heel strike to forefoot strike. More particularly, known devices typically provide cushioning in a strict vertical range of movement. In other words, during cushioning, the rear part of the foot is allowed to move perpendicular to the sole, but not forward, rearward, or inside or out relative to the sole. The runner's body weight, however, is rarely positioned perpendicular to the sole. Consequently, if the foot applies a force in a direction other than perpendicular to the sole, this force may not be adequately cushioned, and the shoe may therefore cause a shock to the body. These shocks, caused by failure to provide a multi-directional range of cushioning, can cause trauma.
A full range of cushioning is particularly important for runners, because a runner's stride constantly changes with speed, distance, change of running surface, and the like. This, in turn, constantly changes the angle between the shoe and leg at impact, the leg's range of rotation, body weight positioning, and the like. Consequently, it is desirable that a running shoe supply cushioning and shock absorption from heel impact until at least forefoot impact (throughout leg rotation), regardless of these changing parameters.
Moreover, during the transition between initial foot strike and forefoot strike, the runner's foot pronates. This is a normal occurrence that allows the foot to act as a natural shock-absorber. The feet of many runner's, however, have abnormal pronation (over or under pronation) which can cause trauma, such as shin splints. Most abnormal pronation problems are due to over pronation, which, in many instances, is caused by flat feet. Under pronation, on the other hand, is usually caused by high arches. In either case, the body's natural shock absorption is reduced, which can lead to trauma. Consequently, there is a need to place a runner's foot in a neutral position in a shoe, which reduces abnormal pronation problems.
People with abnormal pronation, however, have walked and run for years with their feet in non-neutral positions. Forcing such a runner's foot into a neutral position without any transition period can actually cause discomfort and trauma. Consequently, there is a need to correct for abnormal pronation, and also a need to correct for abnormal pronation over a period of time to allow for transition of foot placement.